This invention relates to the treatment for the surface of a substance being treated by use of plasma, and particularly to a method of supplying a bias voltage to a substance of which the surface is to be treated.
FIG. 2 shows the most typical conventional biasing method called the RF biasing. A substance 1 to be etched is connected through a capacitor 2 to a high-frequency power supply 3. A sine-wave voltage as shown in FIG. 3 is applied through the capacitor 2 to the substance 1. At this time, since the amount of electrons which a plasma 4 supplies is dozens of times larger than that of ions, negative charges are accumulated on the substance-side of the capacitor 2. Because of the capacitor charges, a voltage shifted in the negative direction as shown in FIG. 4 appears on the substrate. The positive ions for etching are accelerated by this negative voltage and perpendicularly incident on the substrate, thus making it possible to etch the substrate in an anisotropic shape.
Another idea is disclosed in JP-A-56-13480 (laid-open on Feb. 9, 1981) and JP-A-6-61182 (laid-open on Mar. 4, 1994) in which a pulse-waveform voltage is used as a bias voltage. However, the importance of the duty ratio and repetition frequency of a pulse wave has never been considered so far except that the present invention regards them as important.